记法 : 次级等于上级
TARGET_OS_WIN32 - Generated code will run under 32-bit Windows
TARGET_OS_UNIX - Generated code will run under some Unix (not OSX)
TARGET_OS_MAC - Generated code will run under Mac OS X variant/对于（可能）在Apple平台上运行的任何Cocoa应用程序，TARGET_OS_MAC将为1。
TARGET_OS_OSX - Generated code will run under OS X devices
TARGET_OS_IPHONE - Generated code for firmware, devices, or simulator
TARGET_OS_IOS - Generated code will run under iOS
TARGET_OS_TV - Generated code will run under Apple TV OS
TARGET_OS_WATCH - Generated code will run under Apple Watch OS
TARGET_OS_BRIDGE - Generated code will run under Bridge devices
TARGET_OS_SIMULATOR - Generated code will run under a simulator
TARGET_OS_EMBEDDED - DEPRECATED: Use TARGET_OS_IPHONE and/or
TARGET_OS_SIMULATOR instead
TARGET_IPHONE_SIMULATOR - DEPRECATED: Same as TARGET_OS_SIMULATOR
TARGET_OS_NANO - DEPRECATED: Same as TARGET_OS_WATCH